Optical sensors are used in a wide variety of applications. One application where optical sensors are of particular use is to detect the presence and/or absence of an object. Two types of optical sensors that detect the presence and absence of an object are interrupt optical sensors and reflective optical sensors.
Interrupt optical sensors (also known as transmissive sensors) include an optical emitter and an optical detector located on opposite sides of an object to be sensed. The presence of the object is detected when the object breaks the optical path between the emitter and the receiver. The absence of the object is shown when the optical path between the emitter and of the receiver remains undisturbed (i.e., since the object does not break that optical path).
Reflective optical sensors, on the other hand, contain an optical emitter and an optical detector that are located on the same side of the object to be sensed. The object is detected when an optical emission from the optical emitter is reflected from a surface of that object and then received by the optical detector. Reflective optical sensors typically contain a light emitting diode (“LED”) as the optical emitter and a photodiode or phototransistor as the optical detector. These two components are mounted in a side-by-side relationship in the housing of the optical sensor. The LEDs can have a broad angular emission window projected in a large undefined angular range (i.e., similar to a conventional light bulb).
The performance of optical reflective sensors can be measured, in part, by the contrast provided when scanning an object that contains alternating reflective and non-reflective surfaces. This contrast is typically determined by the reflectance of those surfaces, the resolution of the sensor compared to the dimensions of the object, and the amount of cross talk that contributes to the photocurrent between the optical emitter and the optical receive detector. This cross talk is the portion of emitted light that it internally reflected, scattered, or channeled to the optical detector (as opposed to that external light which is reflected by the object), as well as any direct illumination from the LED. Generally, the smaller the cross-talk, the better the operation of the optical reflective sensor.
There are currently two methods to minimize the problems associated with cross talk. As shown in FIG. 1, the first method places an opaque barrier 116 between the emitter (located in a first cavity 112) and the detector (located in a second cavity 114) in the sensor 100, and so is often called a packaging solution. This first method, while somewhat effective in preventing cross talk, requires a complex assembly process and does not allow for sophisticated optics.
The second method for minimizing cross talk processes the light signal in a differential manner by using a dual photodiode and a comparator. This method is therefore often called a die design solution. The die design solution is not very effective since the light is not evenly spread over the 2 photodiodes, does not work for analog output, and is expensive as it doubles the size of the photosensitive area.